The present invention relates to a checking apparatus for a master film etc. for a printing plate, and more particularly to a checking apparatus for determining whether or not a new film has errors, by comparing an uncorrected old film and the corrected new film.
In a conventional photomechanical process, various materials such as a film obtained by subjecting a color picture original film to color separation and a halftone process, a character original formed by a computer type setting device, a tint, and a lightproof mask film are adhered on a transparent film in accordance with a predetermined layout and in units of colors. The obtained films are multi-exposed and nega/posi or posi/nega inverted to obtain four master films of yellow (Y), magenta (M), cyan (C), and black (BK). In offset printing, the master films are placed on a pre-sensitized plate (PS plate) and the PS plate is exposed to form a printing plate. A proof is printed from the printing plate and subjected to client proofreading.
The master films must be checked and corrected before forming the printing plate and after the proofreading. For example, they must be checked to determine whether the error has been corrected, whether the color separation films are registered among the four color master films, and whether the tint and characters are arranged at predetermined designated positions. The master films may sometimes have to be formed again if there are many errors. Also, the first master films containing errors may be compared with revised master films. In this case, since the master films are formed manually and in complex procedures and the proofreading is a manual work, a portion of a first film other than the defective or designated portion can be erroneously corrected in a second film.
A hard copy of, e.g., a rough proof is conventionally prepared for the master film checking. However, it increases running costs such as material costs.
In order to solve this problem, one of the present inventors has proposed a checking apparatus described in "Previewing apparatus of flats " (U.S. Pat. No. 4,463,373). According to this apparatus, four master films are placed on a table. Optical images of light emitted from a light source under the table and transmitted through the master films are picked up at once. The obtained transmittance optical images of the master films of the respective colors are colored in corresponding colors, superposed on each other, and displayed at a single display screen. As a result, it can be checked on the display screen whether the master films of the respective colors are well-registered, and whether the tint and characters are arranged at predetermined designated position in the respective colors.
With the conventional apparatus, however, since the light source is provided only under the table, only a transmission-type object, i.e., a master film can be picked up. Thus, a relfection-type object, such as a blue print formed from a master film, a photocopy, a proof, and a block copy cannot be photographed. A printout sample, however, must be corrected several times during printing, and the printout sample must also be checked several times. Therefore, it is inconvenient if only a transmission-type object, e.g., a master film can be picked up.
Furthermore, according to the conventional apparatus, four images are merely superposed and displayed. Although the registration between the films can be checked, it is difficult to check whether a portion to be changed is corrected as designated.
A large space is required for the conventional apparatus since four color films of Y, M, C, and BK are held on a single plane. Since the relative positional precision among the four TV cameras must be high, the apparatus becomes expensive. Since the TV cameras correspond to Y, M, C, and BK, respectively, the master films must be placed at corresponding predetermined positions.
The white and black levels of the respective TV cameras must be adjusted to be the same. Conventionally, however, a level of an output signal of a TV camera is displayed on an oscilloscope and the white and black levels are adjusted while monitoring the oscilloscope screen. Therefore, adjustment operation is complex, and a space for an oscilloscope is needed.